Graphene is an atomically thin, two-dimensional carbon material with highly desirable properties including a large specific surface area, impressive mechanical properties, and high thermal and electrical conductivities. These exceptional properties find widespread applications in diverse fields including catalysis, composites, energy storage and biomedical scaffolds. However, graphene's propensity toward aggregation and restacking can significantly degrade device performance.
An essential prerequisite for graphene's widespread application is the controlled large-scale assembly of two-dimensional graphene building blocks into three-dimensional (3D) structures while maintaining exceptional properties (e.g., large surface area, mechanical properties, etc.). A variety of 3D graphene-based materials are being explored which focus on providing a network of interconnected pores in order to minimize stacking and fully exploit graphene's properties. Within these materials, control of pore morphology and size is critical in order to obtain the desired material properties.
Several synthesis methods for 3D graphene have been proposed, both with and without template guides. For template-guided methods, such as chemical vapor deposition (CVD) coatings on metallic foams, processing has not been scalable, and the materials obtained from these methods are generally brittle under low compression. Template-free approaches are more scalable and versatile with synthesis capable of a wide-range of pore morphologies including ultrafine (<100 nm) to macro (>1 μm).
Chemically derived graphene oxide (GO) based aerogels are the most common 3D graphene in the literature. This method relies on self-assembly or gelation of the GO suspension via hydrothermal reduction, chemical reduction, or direct cross-linking of the GO sheets. Other methods, particularly ice-templating has demonstrated some control over pore morphology; however, the architecture remains largely stochastic resulting in limited mass transport and non-optimal mechanical properties. Thus, the fabrication of 3D graphene materials with tailored macro-architectures via a controllable and scalable assembly method is still a significant challenge.
The properties of cellular solids are largely determined by their chemical composition, porosity, and cell morphologies. In recent years, additive manufacturing (AM) techniques have been utilized to make architected and hierarchical cellular materials structured from the nanometer to the centimeter scale resulting in unique properties otherwise unattainable. Recently, a 3D printing technique based on extrusion known as direct-ink writing (DIW) was utilize to fabricate highly compressible graphene aerogel microlattices and supercapacitors. These graphene aerogels showed improved mechanical strength beyond most bulk stochastic graphene assemblies while maintaining the large surface area of single graphene sheets. However, the DIW technique has limits in materials, scaling, and speed due to its serial nature. Its inks require a gel-based viscoelastic sheer thinning behavior typically accomplished by incorporating diverse fillers which profoundly impact final material processing and performance. Current processes are limited to log-pile like objects, and not true free-form fabrication, severely limiting the potential for the materials.